docs: def _decode_exp(self, access_token=None):
          """Extract exp field from access token.
  
          Args:
              access_token (str): Access token to decode. Defaults to ``None``.
  
          Returns:
              int: JWT expiration in epoch seconds.
  
          """
          c = self.get_credentials()
          jwt = access_token or c.access_token
          x = self.decode_jwt_payload(jwt)
  
          if 'exp' in x:
              try:
                  exp = int(x['exp'])
              except ValueError:
                  raise PanCloudError(
                      "Expiration time (exp) must be an integer")
              else:
                  self.jwt_exp = exp
                  return exp
          else:
              raise PanCloudError("No exp field found in payload")